Accidentally Painting Normal Map
When painting, you need to choose which image is active by selecting it in the Slots tab of the toolbar. Normal maps work on colors that represent xyz axis for normals, and bump maps use black to white gradient for in and out displacements. Reordering the textures in the Properties texture stack won't affect it unless both are set to 'color' and then the one on the bottom of the stack blocks out the one on the top of the stack.